麻煩三重加入

Question

我有三個表：

訂單

orderID 
userID 

用戶

userID 
lastViewedOrderID 

votehistory

userID 
orderID 

我有一個PHP函數：getOrdersForUser($userID)，它應該返回一組的orderID值與下列條件：

1）用戶還未觀看的訂單（orders.orderID> users.lastViewedOrderID）

2）的命令未被列入由用戶正在查詢（orders.userID！= users.userID）

3）尚未由用戶根據所述votehistory表表決（的orderID不在votehistory其中userID = [傳入的用戶ID值]）

到目前爲止，我想出最好的是：

SELECT orders.orderID 
FROM orders 
JOIN users 
ON users.userID != orders.userID 
JOIN votehistory 
ON (votehistory.userID = users.userID) AND (votehistory.orderID != orders.orderID) 
WHERE users.userID = [the userID value passed in] 
AND orders.orderID > users.lastViewedOrderID 
AND likehistory.orderID != orders.orderID 

不幸的是，這給了我一噸重複 - 同樣的orderID重複在votehistory每個有效行。我似乎不瞭解JOIN過程如何優化。我應該嘗試減少通過子查詢連接的表中的行數？什麼是排除重複的最有效方法？

Answer 1

增加了一些指標之後，我發現這個查詢的工作效率：

SELECT orders.orderID 
FROM orders 
WHERE orders.orderID NOT IN 
( 
    SELECT votehistory.orderID 
    FROM users 
    JOIN votehistory 
    ON (votehistory.userID = users.userID) 
    WHERE users.userID = [userID input value] 
) 
AND orders.userID != [userID input value] 
AND orders.orderID > 
(
    SELECT lastViewedOrderID 
    FROM users 
    WHERE userID = [userID input value] 
) 
ORDER BY orders.orderID ASC 

Answer 2

因爲你只想要orderID，你可以用EXISTS來做。就像這樣：

SELECT orders.orderID 
FROM orders 
WHERE EXISTS 
( 
    SELECT NULL 
    FROM users 
    JOIN votehistory 
    ON (votehistory.userID = users.userID) 
    WHERE users.userID != orders.userID 
    AND users.userID = [the userID value passed in] 
    AND (votehistory.orderID != orders.orderID) 
    AND orders.orderID > users.lastViewedOrderID 
    AND likehistory.orderID != orders.orderID 
) 

參考：

• 13.2.9.6 Subqueries with EXISTS or NOT EXISTS